How does rising atmospheric CO2 affect marine organisms?

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Eastern Gotland Basin, Baltic Sea
Andrén, E., Andrén, T. and Kunzendorf, H. 2000. Holocene history of the Baltic Sea as a background for assessing records of human impact in the sediments of the Gotland Basin. The Holocene 10: 687-702.

The authors analyzed the organic carbon content (more of which indicates conditions conducive to greater primary productivity) and identified and quantified siliceous microfossil assemblages (certain species of which are indicative of higher temperatures) of a sediment core retrieved in 1997 from a point in the Eastern Gotland Basin (57°16.9772'N, 20°07.1122'E) of the Baltic Sea. This work revealed an increase in organic carbon content that began about 1700 calendar years before present (cal. yr BP) that reached a maximum value about 900-800 cal. yr BP, "pointing to very high primary production at that time." In addition, they found that the diatom assemblage of this high productivity event consisted of "up to 90% Pseudosolenia calcar-avis, a common marine planktonic tropical and subtropical water species which occurs seasonally in temperate waters," but which they say "cannot be found in the present Baltic Sea." Therefore, we conclude that the MWP in this region -- which Andren et al. assign to approximately AD 900-1300 -- was likely significantly warmer than it was over the latter part of the 20th century.